The present invention relates to a vehicle height adjustment device, and in particular to a vehicle height adjustment device which provides suspension for the wheels of a vehicle and which can raise or lower the body of the vehicle in a controlled and regular fashion.
A suspension system for a vehicle in which the height of the various parts of the vehicle body from the ground can be adjusted typically comprises: a source of pressurized working fluid, which may be a substantially incompressible working fluid such as oil or hydraulic fluid, but may alternatively be a compressible working fluid such as compressed air, and which is typically pressure controlled as for example by a pressure control valve; a number of fluid pressure actuated wheel suspension units, one for each of the wheels of the vehicle, each of which when supplied with pressurized working fluid pushes its wheel downwards with respect to the vehicle body and thereby lifts the vehicle body up from the ground; a branching conduit system, which supplies the pressurized working fluid to these wheel suspension units; and a plurality of control valves at various points of this branching conduit system, which according to their selective actuation by a control system for the vehicle height adjustment system selectively provide more or less such pressurized working fluid to the various wheel suspension units by being opened or closed.
In such a type of per se conventional vehicle height adjustment system, however, difficulties tend to arise with respect to smooth and coordinated alteration of the vehicle elevation. Typically, the flow resistances of the various parts of the conduit system which branch to these various wheels of the vehicle will be different, according to their different length; and quite often the flow resistances of the conduits to corresponding wheels on the left and the right of the vehicle, even, will differ. This means that, when the vehicle height is to be altered by raising it for example, at each of the branch points of the branching conduit system where the flow of pressurized working fluid from the source thereof is branched to be supplied to various ones of the wheels or to various combinations thereof, this pressurized working fluid flow will tend to be preferentially directed to that branch of the branching conduit system which provides the lesser flow resistance, and this means that all the wheels of the vehicle are not driven down with respect to its body in unison. This problem of uneven and unpredictable division of the flow of working fluid at each of the branch points of the branching conduit system is aggravated by variations of load on the vehicle and by alteration of the viscosity of the working fluid due to effects of temperature and contamination. Thus, the speed of adjustment of vehicle height may differ from one wheel to another, and from one end of the vehicle, or one side thereof, to the other. This may cause the attitude of the vehicle body to change during the process of alteration of vehicle elevation, which not only can cause anxiety to the driver and passengers of the vehicle, but can cause alteration of the field of vision of the driver, disturbance in the adjustment of the headlamps of the vehicle, deterioration of the drivability and the favorable running conditions of the vehicle, and worsening of the aerodynamic qualities of the vehicle which can damage the fuel economy thereof.
Further, because typically other subsystems of the vehicle other than the suspension system also utilize pressurized working fluid, further in the prior art there is a wasteful duplication of working parts in the vehicle as a whole, because other separate pumps and pressure control valves have been used for providing pressurized working fluid for these other subsystems. Accordingly the compactness and weight of the vehicle, and the reliability thereof, have been deteriorated.